1. Field of the Invention
The present invention relates to a ferroelectric memory, particularly, to a ferroelectric memory utilizing the residual polarization characteristics of an oxide ferroelectric thin film.
2. Description of the Related Art
Ferroelectric compounds exhibit singular electrical characteristics and, thus, are used in various fields including, for example, a piezoelectric filter or an ultrasonic transducer utilizing piezoelectric characteristics, an infrared sensor or an image sensing device utilizing pyroelectricity and a light-modulated element or an optical shutter utilizing electro-optical characteristics. Further, an electronic device utilizing a thin film of a ferroelectric compound has already been developed, and vigorous efforts are being made in an attempt to decrease the thickness of the ferroelectric film. Particularly, a nonvolatile memory device having a ferroelectric thin film capacitor mounted thereon, which utilizes the stability of the residual polarization of the ferroelectric thin film, attracts special attentions nowadays because severe competition is being carried out recently in an attempt to improve the memory capacity and degree of integration of the memory device.
A series of lead-containing complex oxide ferroelectric compounds such as PZT (lead titanate zirconate) and PLZT (lanthanum lead titanate zirconate) are typical materials which are being vigorously studied for practical use in various fields by many researches over many years.
A bismuth stratified perovskite type oxide is another prospective ferroelectric material because a ferroelectric thin film formed of oxide of this type exhibits an excellent fatigue resistance. Specifically, the number of recording/erasing operations achieved by the particular ferroelectric thin film is hundreds of times as many as that for the conventional ferroelectric thin film formed of PZT or PLZT. Thus, the ferroelectric thin film formed of a bismuth stratified perovskite type oxide can be used in a nonvolatile memory device having a high durability and in other electronic and optical devices.
The ferroelectric thin film can be formed by physical film-forming methods such as a sputtering method using as a target a ceramic material baked in advance and a reactive vapor deposition of metal, a CVD method in which an organometallic compound is deposited in a vapor phase, a sol-gel method in which coating of a solution of a ferroelectric compound is utilized for forming a film, a MOD method, etc.
In general, a ferroelectric material exhibits hysteresis characteristics in respect of the residual polarization. The hysteresis characteristics are utilized for storing data in a nonvolatile memory. FIG. 9 shows the construction of a conventional ferroelectric memory 31 using a ferroelectric material. As shown in the drawing, a ferroelectric thin film capacitor 37 is formed on a silicon semiconductor substrate 33 having peripheral elements 32 such as a switching element comprising a MOS transistor, and an amplifier formed therein. The capacitor 37 is formed by successively laminating a lower electrode 34 made of, for example, platinum, a ferroelectric thin film 35 and an upper electrode 36 on the silicon substrate 33. Further, an interlayer insulating film 38 made of silicon oxide is formed by a sputtering method or a CVD method, followed by forming an electrode wiring 39 for connecting the elements to each other.
In addition to the basic structure described above, a protective layer (not shown) is formed, if necessary, so as to protect an under-layer 40 serving to improve the adhesivity of the lower electrode 34 to the substrate and the ferroelectric thin film capacitor from various changes in environment. Some improvements in the protective film of a ferroelectric memory are proposed to date, as exemplified below:
(1) Japanese Patent Disclosure No. 2-183569 teaches that a Ti.sub.3 N.sub.4 layer is formed between a ferroelectric PZT thin film and an upper electrode and, at the same time, the PZT thin film is covered directly with a Si.sub.3 N.sub.4 layer, so as to prevent oxygen from being released from the PZT layer and, thus, to improve the switching fatigue characteristics. PA1 (2) Japanese Patent Disclosure No. 5-90606 teaches that a capacitor upper electrode is formed by laminating a first conductive film consisting of platinum or palladium and a second conductive film consisting of titanium nitride, titanium-tungsten alloy or molybdenum silicide so as to prevent alloying between the aluminum wiring electrode and the first conductive layer and, thus, to make a heat treatment in the subsequent step possible.
However, in the manufacturing process of the conventional ferroelectric memory described above, a ferroelectric thin film formed by a sputtering method or a spin-on method is subjected to a heat treatment at a relatively high temperature of 600.degree. to 900.degree. C. because such a heat treatment is absolutely necessary for the crystallization of the ferroelectric thin film. The manufacturing process also comprises, for example, an ion milling step utilizing a high energy beam and a reactive plasma etching step in which the ferroelectric thin film is exposed to a plasma bombardment. As a result, numerous lattice defects are generated in the single crystal structure of the semiconductor silicon substrate, leading to deterioration in the characteristics of the MOS transistor formed on the substrate.
To overcome the difficulty, a heat treatment (MOS sintering) is performed in the final step at 350.degree. to 450.degree. C. under a hydrogen-nitrogen mixed gas (forming gas) atmosphere. In this heat treatment, defects such as dangling bonds generated in the monocrystalline silicon substrate are eliminated by utilizing the reducing property of the hydrogen gas so as to restore the MOS characteristics.
In the prior art, however, the electrode for the ferroelectric thin film is formed of, for example, platinum which readily transmits hydrogen, giving rise to a serious problem. Specifically, hydrogen is diffused during the heat treatment under a reducing atmosphere through the interlayer insulating film and the upper electrode so as to arrive at the interface between the upper electrode and the ferroelectric thin film and at the side surface of the capacitor. What should be noted is that the oxide in the vicinity of the interface is decomposed by an oxidation-reduction reaction caused by the reducing function of the hydrogen arriving at the interface. In other words, the adhesivity of the ferroelectric thin film to the upper electrode is lowered by the chemical change taking place at the interface. Alternatively, the upper electrode is pushed up by the oxygen gas, water, etc. generated by the oxidation-reduction reaction noted above. It follows that peeling is likely to take place at the interface between the upper electrode and the ferroelectric thin film.
Further, the hydrogen gas is considered to be diffused through the side surface of the capacitor to reach the lower electrode. Thus, in the case of using an Al--Si electrode wiring in which a relatively large internal stress is likely to remain, the entire capacitor tends to be peeled off the semiconductor substrate itself. It is of course important to take measures against the particular problem in forming a ferroelectric memory by combining a ferroelectric thin film capacitor and a silicon semiconductor device.